Solid golf ball

ABSTRACT

A solid golf ball having good controllability and long flight distance which utilizes a solid core and a cover covering the core, wherein the core is formed from a rubber composition comprising a polybutadiene rubber having a cis-1,4-structure content of at least 40% and either an unsaturated carboxylic acid or a metal salt thereof or both, and the cover has a thickness of 1 to 4 mm, a Shore D hardness of 55 to 64 and has 300 to 450 dimples having a diameter of 2 to 5 mm, with the dimples having a dimple area proportion of 62 to 78% and a total dimple volume of 260 to 345 mm 3 .

FIELD OF THE INVENTION

The present invention relates to a solid golf ball having excellentflight performance and excellent spin performance.

BACKGROUND OF THE INVENTION

Hitherto, there have been mainly produced two types of golf balls. Oneis a solid golf ball, such as a two piece golf ball, which comprises acore formed from vulcanized rubber material and a thermoplastic cover(e.g. ionomer cover) formed on the core. The other is a thread woundgolf ball which comprises a liquid or solid center, a thread rubberwinding layer formed on the center and a balata or ionomer cover formedthereon. The solid golf ball, when hit by either a driver or an ironclub, flies a with parabolic trajectory and achieves a long flightdistance, thus showing excellent flight performance, in comparison withthe thread wound golf ball. This feature is probably brought about byits inner structure, i.e. the solid golf ball has such structuralfeatures that it does not achieve much spin and creates a parabolictrajectory which provides much run after landing on the ground therebyproducing greater total flight distance.

On the other hand, when the solid golf ball is hit by an iron club itproduces a so-called flies and thus, it is difficult to stop the ball onthe green, because spin is not easily applied to the ball. Accordingly,professional golfers or high-level golfers dislike the flightperformance of solid golf balls.

OBJECT OF THE INVENTION

Thus, a solid golf ball having good spin performance so that one cantake deadly aim at the pin has been desired. In this case, the longflight distance which is an inherent characteristic of the solid golfball would be maintained. Advantageously, the objective golf ball alsohas a good shot feel.

SUMMARY OF THE INVENTION

The present invention is directed to providing a solid golf ball whichhas good shot feel and excellent spin performance which improvescontrollability of the approach shot, by stopping the ball on the green,but at the same time does not adversely affect the characteristicsinherent in solid golf balls, i.e. a parabolic trajectory and longflight distance. The solid golf ball of the present invention comprisesa solid core and a cover covering the core, wherein the core is formedfrom a rubber composition comprising a polybutadiene rubber having acis-1,4-structure content of at least 40% and either an unsaturatedcarboxylic acid or a metal salt thereof or the both, and the cover has athickness of 1 to 4 mm, a Shore D hardness of 55 to 64 and has 300 to450 dimples having a diameter of 2 to 5 mm, and the dimples have adimple area proportion of 62 to 78% and a total dimple volume of 260 to345 mm³.

BRIEF EXPLANATION OF DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is a schematic cross section illustrating one dimple of the solidgolf ball of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

First of all, some mechanism of the present invention will be explained,although it is not limited thereto. When considering the mechanism forthe generation of spin with golf balls, if the same materials and thesame shot conditions are used and if the hardness of the golf ball isonly varied, the amount of deformation of the golf ball is merelychanged by the hardness of the ball, since the coefficient of frictionbetween the golf club face and the ball surface is the same because thesame materials are used. The change in the deformation amount of thegolf ball leads to the difference in distance between the gravity centerof the golf ball and the contact point of the ball with the club face.When the ball is harder, the distance between the gravity center and thecontact point is larger and if the ball is softer, the distance isshorter. This means that the harder the golf ball, the easier it is toapply spin.

Accordingly, in order to increase the spin amount, it is naturallyconsidered that the ball hardness should be made higher. On the otherhand, an increase in the ball hardness makes the shot feel poor.Although it can be possible that the cover of the golf ball is madesofter to compensate for the hard shot feel, the resulting golf ball haspoor impact resilience and poor initial velocity when hit, thusdecreasing the flight distance.

In the present invention, the cover has a Shore D hardness of 55 to 64,preferably 61 to 64 which is slightly lower than that of theconventional two piece solid golf ball. This prevents the ball frombecoming too soft which desperately reduces the initial velocity but hasa high spin amount. It is also known in the art that the enhancement ofthe spin performance adversely reduces the launch angle when strikingthe ball and that the ball is launched lower at first and is then blownup at the end without extending the distance, thus resulting in a poorflight performance. In order to solve the flight performance, thepresent inventors have found that the dimples formed on a surface of thecover are limited to the diameter of 2 to 5 mm and in number from 300 to450 preferably 340 to 390. Furthermore, the dimple area proportion of70±8% of the total surface area and the total dimple volume is 260 to345 mm³, preferably 280 to 330 mm³. These features of the presentinvention provide a trajectory with an extended flight distance.

The solid golf ball of the present invention can be either a two piecesolid golf ball or a multi-piece solid golf ball in which the core orthe cover is made of a plurality of layers. The core and cover can bemade from any materials which have been used for golf balls, as long asthe golf ball satisfies the features as claimed. Typical examples of thecore and cover are hereinafter explained.

The core employed in the solid golf ball of the present invention can beobtained by vulcanizing a rubber composition in a mold. The rubbercomposition used for the core generally contains a base rubber, acrosslinking agent, a co-crosslinking agent, an inert filler and thelike.

The base rubber can be natural rubber or synthetic rubber which has beenused for solid golf balls, for example polybutadiene, polyisoprenerubber, styrene-butadiene rubber and EPDM. Preferred is polybutadienerubber having cis-1,4 structure of at least 40%. The base rubber can bea mixture of the rubbers mentioned above.

The crosslinking agent which is used for the initiating crosslinkingreaction can be peroxides, such as dicumyl peroxide and di-t-butylperoxide. The preferred crosslinking agent is dicumyl peroxide. Theamount of the peroxide is not limited but can be 0.5 to 3.0 parts byweight, preferably 0.8 to 1.5 parts by weight, based on 100 parts byweight of the base rubber.

The co-crosslinking agent is used for inserting a crosslinked structureinto rubber molecules and can be any one which has been used for solidgolf balls. Typical examples of the co-crosslinking agents are metalsalt of unsaturated fatty acid, such as one or divalent metal salt ofα,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. The metalincludes sodium, potassium, magnesium, zinc and the like, and theα,β-unsaturated carboxylic acid includes acrylic acid and methacrylicacid. A preferred co-crosslinking agent is zinc acrylate because itimparts high rebound characteristics to the resulting golf ball. Theco-crosslinking agent can be present in the rubber composition in anamount of 24 to 38 parts by weight, preferably 28 to 34 parts by weightbased 100 parts by weight of the base rubber. Amounts of more than 38parts by weight make the core too hard and lower the shot feel andamounts of less than 24 parts by weight reduce impact resilience.

The inert filler can be one used for golf balls and includes zinc oxide,barium sulfate, silica, calcium carbonate or zinc carbonate. Generallyused is zinc oxide. An amount of the filler is not limited and can varydepending on specific gravity of core and weight regulation of the golfball, but may be within the range of 10 to 60 parts by weight based on100 parts by weight of the base rubber.

The rubber composition can contain other components which have been usedfor cores of golf balls, such as an antioxidant.

The above mentioned components are mixed to form a rubber compositionwhich is then vulcanized at an elevated temperature under pressure in amold to form a solid core. The vulcanization may be conducted at 130° to180° C. for 10 to 60 minutes. The solid core of the present inventionpreferably has a diameter of 37 to 40 mm.

When the core is made of two layers, an inner core is generally madefrom the above mentioned rubber composition and the outer core can bemade from either the above mentioned rubber composition or anotherthermoplastic resin. The inner core preferably has a diameter of 27.0 to38.0 mm, more preferably 28.0 to 36.0 mm and the outer layer has athickness of 0.5 to 6.5 mm, preferably 1.5 to 5.5 mm, so that the totalhas a diameter of 37 to 40 mm. The core can be made of more than twolayers.

The solid core of the present invention preferably has a deformationamount of 2.30 to 3.50 mm, which is determined by applying from aninitial load of 10 Kg to a final load of 130 Kg. If it is less than 2.30mm, the core is too hard. If it is more than 3.50 mm, the core is toosoft. The solid core of the present invention preferably has a JIS-Chardness and has a JIS-C hardness distribution within 5. The JIS-Chardness substantially equals to Shore C hardness and is measuredaccording to JIS K 6301.

The solid core obtained above is covered with a cover. The cover can bemade from any material which has been used for the covers of golf balls,and typical examples of them are ionomer, polyamide, polyester, and amixture thereof. Preferred is ionomer resin. Examples of the ionomerresin which is commercially available from Mitsui Du Pont PolychemicalCo., Ltd. are ionomer resins such as Hi-milan 1557 (Zn), Hi-milan 1605(Na), Hi-milan 1650 (Zn), Hi-milan 1707 (Na), Hi-milan AM7318 (Na),Hi-milan 1705 (Zn), Hi-milan 1706 (Zn), Hi-milan 1652 (Zn), Hi-milanAM7315 (Zn), Hi-milan AM7317 (Zn), Hi-milan AM7311 (Mg), Hi-milan MK7320(K), Hi-milan 8120 (Na); and terpolymer copolymer ionomer resins such asHi-milan 1856 (Na), Hi-milan 1855 (Zn), Hi-milan AM7316 (Zn), etc.Examples of the ionomer resin which is commercially available from DuPont Co., U.S.A. include ionomer resins such as Surlyn 8920 (Na), Surlyn8940 (Na), Surlyn AD8512 (Na), Surlyn 9910 (Zn), Surlyn AD8511 (Zn),Surlyn 7930 (Li), Surlyn 7940 (Li); and terpolymer copolymer ionomerresins such as Surlyn AD8265 (Na), Surlyn AD8269 (Na), etc. Examples ofthe ionomer resin which is commercially available from Exxon ChemicalCo. include lotek 7010 (Zn), 8000 (Na), etc. In addition, Na, Zn, K, Li,Mg, etc., which are described in parenthesis following the trade name ofthe above ionomer resin, mean neutralizing metal ion species thereof.The most preferred combination of the ionomer resins is a mixture of 15to 40% by weight of an ionomer resin having a Shore D hardness of 65 to68, 20 to 40% by weight of an ionomer resin having a Shore D of 60 to 64and 30 to 60% by weight of an ionomer resin having a Shore D hardness of50 to 59; a total weight of the ionomer resins being 100% by weight.

The cover is mainly made from the thermoplastic resin as mentionedabove, but may contain a small amount of additives, such as a colorant(e.g. titanium oxide), a UV absorber, a light stabilizer, a fluorescentagent and a fluorescent brightener, as long as the addition of theadditives does not deteriorate the desired performance of the golf ballcover.

The method of covering the cover on the solid core is not specificallylimited. For example, a method comprising molding a cover compositioninto a semi-spherical half-shell in advance, covering a core with twohalf-shells and then subjecting it to a molding pressure at 130° to 170°C. for 1 to 15 minutes, or a method comprising injection molding thecover composition directly on the core to cover the core is used. Whenmolding the cover, dimples may be optionally formed on the coversurface. After molding the cover, paint finishing and stamping may beoptionally conducted. The cover may be made of two or more layers, usingdifferent cover materials.

According to the present invention, the cover is required to have aShore D hardness of 55 to 64, preferably 61 to 64. The shore D hardnessis measured according to ASTM D-2240. If the cover has a Shore Dhardness of less than 54, rebound characteristics are deteriorated andflight distance is poor. If it is more than 64, the controllability ofthe golf ball is deteriorated and shot feel is also hard. The cover ofthe solid golf ball of the present invention preferably has a thicknessof 1.0 to 4.0 mm, preferably 1.3 to 2.5 mm. If it is more than 4.0 mm,rebound characteristics are poor and flight distance is poor.

The term "dimple area proportion" employed herein means a proportionexpressed by percentage wherein the total of an area of a circle formedby an edge 1 of a dimple 2 is divided by the ball surface areacalculated from the ball diameter (see FIG. 1). The term "total dimplevolume" used herein means the total volume in mm³ of a hollow (obliqueline portion 3 in FIG. 1) formed by a plane 4 in touch with the edge 1of the dimple 2 and an inner wall of the dimple 2. In this context, theterm "total" means the sum of the volume of all of the dimples.

The dimple area proportion of the solid golf ball of the presentinvention is within the range of 62 to 78%. If it is less than 62%, theball does not extend its flight distance. If it is more than 78%, theball is liable to fly too high thus and has poor flight distance. Thetotal dimple volume is preferably within the range of 280 to 330 mm₃. Ifit is less than 280 mm³, the ball is liable fly too high and exhibitspoor flight distance. If it is more than 330 mm₃, the ball does not flyhigher and shows poor flight distance.

The size and weight of the solid golf ball of the present invention isadjusted to a suitable range which satisfies golf rules.

EXAMPLES

The following Examples and Comparative Examples further illustrate thepresent invention in detail but are not to be construed to limit thepresent invention to their details.

Examples 1-3 and Comparative Examples 1-6

Preparation of core

The ingredients shown in Table 1 were sufficiently mixed and vulcanizedinto a spherical core having a diameter of 39.1 mm. The vulcanizationwas conducted as shown in Table 1 in two step and the first step was at145° C. for 20 minutes and the second step was at 168° C. for 10minutes. Table 1 shows JIS-C hardnesses at center and on surface and adeformation amount of from 10 Kg to 130 Kg of the core. ComparativeExample 6 was an example of commercially available balata covered threadwound golf balls (available from Sumitomo Rubber Industries, Ltd. asRoyal Maxfli (black)).

                  TABLE 1                                                         ______________________________________                                               Examples   Comparative Examples                                        Ingredients                                                                            1      2      3    1    2    3    4    5                             ______________________________________                                        BR-01*.sup.1                                                                           100    100    100  100  100  100  100  100                           Zinc acrylate                                                                          35     31     30   31   35   30   35   31                            zinc oxide                                                                             16     17.5   17.5 17.5 17   18   17   20                            Antioxidant*.sup.2                                                                     --     0.5    0.5  0.5  --   --   --   0.5                           Dicumyl per-                                                                           1      1.2    1.2  1.2  1    1    1    1.2                           oxide                                                                         JIS-C hardness                                                                         76     77     75   77   76   72   76   77                            at center                                                                     JIS-C hardness                                                                         79     78     78   78   79   78   79   78                            on surface                                                                    Deformation                                                                            2.65   2.75   2.85 2.75 2.65 2.85 2.65 2.75                          amount of core                                                                (mm)                                                                          ______________________________________                                         *.sup.1 Polybutadiene rubber having a cis1,4 content of 97%, available        from Japan Synthetic Rubber Co., Ltd.                                         *.sup.2 Available from Ouchi Shinko Kagaku K.K.                          

Preparation of golf ball

The ingredients shown in Table 2 was mixed and injection-molded onto thecore obtained above to form a cover layer. Shore D hardness and coverthickness of each cover were determined and the result are shown inTable 2.

                  TABLE 2                                                         ______________________________________                                                Examples   Comparative Examples                                               1    2      3      1    2    3    4    5                              ______________________________________                                        Hi-milan 1707*.sup.3                                                                    30     --     20   --   30   --   30   --                           Hi-milan 1706*.sup.4                                                                    --     30     --   30   --   50   --   --                           Hi-milan 1557*.sup.5                                                                    --     30     30   30   --   --   --   --                           Hi-milan 1855*.sup.6                                                                    40     40     50   40   40   --   40   --                           Hi-milan 1652*.sup.7                                                                    --     --     --   --   --   --   --   50                           Hi-milan 1650*.sup.8                                                                    30     --     --   --   30   --   30   --                           Hi-milan 8120*.sup.9                                                                    --     --     --   --   --   --   --   50                           Hi-milan 1605*.sup.10                                                                   30     --     --   --   30   50   30   --                           Shore D   61     62     61   62   61   68   61   53                           hardness                                                                      Cover thickness                                                                         1.6    1.9    2.4  1.9  2.1  2.1  2.1  4.5                          (mm)                                                                          ______________________________________                                         *.sup.3 Ionomer resin neutralized with Na, available from Mitsui DuPont       Chemical Co., Ltd.                                                            *.sup.4 Ionomer resin neutralized with Zn, available from Mitsui DuPont       Chemical Co., Ltd.                                                            *.sup.5 Ionomer resin neutralized with Zn, available from Mitsui DuPont       Chemical Co., Ltd.                                                            *.sup.6 Ionomer resin neutralized with Zn, available from Mitsui DuPont       Chemical Co., Ltd.                                                            *.sup.7 Ionomer resin neutralized with Zn, available from Mitsui DuPont       Chemical Co., Ltd.                                                            *.sup.8 Ionomer resin neutralized with Zn, available from Mitsui DuPont       Chemical Co., Ltd.                                                            *.sup.9 Ionomer resin neutralized with Na, available from Mitsui DuPont       Chemical Co., Ltd.                                                            *.sup.10 Ionomer resin neutralized with Na, available from Mitsui DuPont      Chemical Co., Ltd.                                                       

Dimples were formed as the cover was injection-molded. Number ofdimples, dimple area proportion and total dimple volume were measuredand calculated and the result are shown in Table 3.

The resulting solid golf balls were subjected to the evaluation of ballweight, ball deformation amount, ball initial velocity when hit by adriver, flight distance when hit by a driver, spin amount when hit by adriver, height of trajectory (launch angle), spin amount when hit by asand wedge and controllability of approach shot conducted byprofessional golfers, and the results are shown in Table 3.

Ball deformation amount: A deformation of a golf ball when applying froman initial load of 10 Kg to a final load of 130 Kg.

Initial velocity, flight distance, spin amount and launch angle A golfclub was attached to a swing robot available from True Temper Co. andgolf balls were hit. When a driver (No. 1 wood available from CallawayCo., as Great Big Birtha) was used, its head speed was 45 m/second, andwhen a sand wedge (available from Callaway Co., as Big Birtha Iron) wasused, its head speed was 20 m/second. In case of the driver, a flightdistance was determined from the hit point to the point where the ballfirstly reached on the ground. Spin amount, initial velocity and lauchangle were determined by a photograph which was taken immediate afterhitting the golf ball.

Controllability at approach shot: 10 professional golfer hit golf balls.The approach shot was a shot toward a green from a distance of 10 to 70yards, using a sand wedge. Evaluation is as follow.

Excellent=Stop on the green very good

Good=Stop on the green a little

Fairly good=Ordinary

Poor=Difficult to stop on the green

    __________________________________________________________________________                  Examples    Comparative Examples                                              1   2   3   1   2   3   4   5   6                               __________________________________________________________________________    Number of dimples                                                                           342 360 372 360 286 480 432 360 432                             Dimple area proportion (%)                                                                  70  72  66  62  55  82  76  72  78                              Total dimple volume (mm.sup.3)                                                              305 300 295 310 295 305 275 300 330                             Ball weight (g)                                                                             45.3                                                                              45.4                                                                              45.3                                                                              45.4                                                                              45.3                                                                              45.3                                                                              45.5                                                                              45.3                                                                              45.3                            Ball deformation amount (mm)                                                                2.5 2.5 2.55                                                                              2.5 2.4 2.3 2.4 2.3 --                              Initial velocity of ball (m/sec)                                                            65.5                                                                              65.4                                                                              65.6                                                                              65.4                                                                              65.5                                                                              66.1                                                                              65.6                                                                              64.1                                                                              --                              Flight distance by driver (yards)                                                           221.5                                                                             221.7                                                                             221.0                                                                             219.3                                                                             218.7                                                                             22.1                                                                              217.5                                                                             217.9                                                                             --                              Spin amount by driver (rpm)                                                                 3200                                                                              3250                                                                              3150                                                                              3250                                                                              3200                                                                              3080                                                                              3400                                                                              3450                                                                              --                              Lauch angle (°)                                                                      14.2                                                                              14.4                                                                              14.3                                                                              13.9                                                                              14.1                                                                              13.9                                                                              15.8                                                                              14.2                                                                              --                              Spin amount by sand wedge                                                                   5700                                                                              5500                                                                              5650                                                                              5500                                                                              5750                                                                              4400                                                                              5700                                                                              5350                                                                              5800                            (rpm)                                                                         Controllability at approach                                                                 Excel-                                                                            Excel-                                                                            Excel-                                                                            Excel-                                                                            Excel-                                                                            Poor                                                                              Good                                                                              Fairly                                                                            Excel-                                        lent                                                                              lent                                                                              lent                                                                              lent                                                                              lent        good                                                                              lent                            __________________________________________________________________________

The solid golf ball of Comparative Example 3 has a Shore D hardness of68 in cover and many dimples on the surface. The golf ball belongs tothe conventional two piece solid golf ball. The solid golf balls ofExamples 1 to 3 have a little lower initial velocity than the ball ofComparative Example 3, but its flight performance nearly equals to theconventional golf ball. The other properties, i.e. spin performance andcontrollability at approach of Examples 1 to 3, are superior toComparative Example 3.

The golf ball of Comparative Example 1 has a lower dimple areaproportion and therefore shows poor flight distance.

The golf ball of Comparative Example 2 has a less number of dimples andtherefore shows poor flight distance.

The golf ball of Comparative Example 4 has a higher dimple areaproportion and is liable to be blown up, thus not extending flightdistance.

The golf ball of Comparative Example 5 has a thicker cover layer and alower cover hardness, and therefore has a lower initial velocity andpoor flight distance.

What is claimed is:
 1. A solid golf ball having a good flightperformance and a good spin performance comprising a solid core and acover covering said core, wherein said core is formed from a rubbercomposition comprising a polybutadiene rubber having a cis-1,4-structurecontent of at least 40% and either an unsaturated carboxylic acid or ametal salt thereof or the both, and the cover has a thickness of 1 to 4mm, a Shore D hardness of 55 to 64 and has 300 to 450 dimples having adiameter of 2 to 5 mm, and the dimples have a dimple area proportion of62 to 78% and a total dimple volume of 260 to 345 mm³.
 2. The solid golfball according to claim 1, wherein said core has a deformation amount offrom 2.30 to 3.50 mm by applying a load of from 10 Kg load to 130 Kg. 3.The solid golf ball according to claim 2, wherein said core has a JIS-Chardness of 50 to 85 and a JIS-C hardness distribution within
 5. 4. Thesolid golf ball according to claim 2, wherein said cover is formed froma mixture of 15 to 40% by weight of an ionomer resin having a Shore Dhardness of 65 to 68, 20 to 40% by weight of an ionomer resin having aShore D of 60 to 64 and 30 to 60% by weight of an ionomer resin having aShore D hardness of 50 to 59; a total weight of the ionomer resins being100 by weight.
 5. The solid golf ball according to claim 1, wherein saidcore has a JIS-C hardness of 50 to 85 and a JIS-C hardness distributionwithin
 5. 6. The solid golf ball according to claim 5, wherein saidcover is formed from a mixture of 15 to 40% by weight of an ionomerresin having a Shore D hardness of 65 to 68, 20 to 40% by weight of anionomer resin having a Shore D of 60 to 64 and 30 to 60% by weight of anionomer resin having a Shore D hardness of 50 to 59; a total weight ofthe ionomer resins being 100% by weight.
 7. The solid golf ballaccording to claim 1, wherein said cover is formed from a mixture of 15to 40% by weight of an ionomer resin having a Shore D hardness of 65 to68, 20 to 40% by weight of an ionomer resin having a Shore D hardness of60 to 64 and 30 to 60% by weight of an ionomer resin having a Shore Dhardness of 50 to 59; the total weight of the ionomer resins being 100%by weight.
 8. The solid golf ball of claim 1, wherein the rubbercomposition contains a crosslinking agent in an amount of 0.5 to 3.0parts by weight based on 100 parts by weight of the base rubber.
 9. Thesolid golf ball of claim 8 wherein the rubber composition contains aco-crosslinking agent in an amount of 24 to 38 parts by weight based on100 parts by weight of the base rubber.
 10. The solid golf ball of claim1, wherein the total dimple volume is 280 to 300 mm³.
 11. The solid golfball of claim 1, wherein the ball weight is 45.3 to 45.4 g.